The invention concerns a plate, for example a motor vehicle licence plate.
One form of motor vehicle licence plate as is to be found in German utility model No 297 12 954.6 comprises a casing which has a flat rear wall and a forwardly projecting rim which extends around the edge of the rear wall. The rim defines a light transmission opening, through which it is possible to see a flat symbol carrier which extends parallel to the rear wall of the casing. The symbol carrier has regions of varying transparency to represent symbols thereon. To provide light between the rear wall and the symbol carrier, the plate has an electrically activatable light film which shines light through at least the regions of high transparency of the symbol carrier, from the rear thereof. The symbol carrier is formed by a stiff plastic plate member arranged in the casing which serves as a form of holding frame. The light film can be more particularly in the form of an electroluminescence film. The light transmission opening through which the light produced by the light film when activated issues to the exterior after having shone through the symbol carrier from the rear does not have any further closure so that the symbol carrier can be readily held securely by the projecting rim of the casing, by virtue of the stiffness of the symbol carrier, and thereby also fixes the light foil which is disposed behind it.
That design configuration enjoys advantages insofar as the electrically activatable light film provides for direct lighting of the background of the identification symbols without any need for a separately arranged lighting system. This means that the licence plate can be of an extremely compact and space-saving configuration.
It does however suffer from the disadvantage that there are no stiff plastic plate members, that is to say which are suitable as a closure for the light transmission opening, which are not only transparent for the light produced by the light foil and thus coming from the rear, while at the same time also being retroreflective for extraneous light which is incident on the symbol carrier from the exterior. This is a factor which is generally required by legislation in this respect, in particular for motor vehicle licence plates, but which is also an advantageous consideration or is required for other plates, for example traffic signs.
An object of the present invention is to provide a plate such as a vehicle licence plate which combines incorporated lighting by means of an electrically activatable light film and retroreflective properties.
Another object of the present invention is to provide a plate which incorporates its own lighting means and also affords retroreflective properties while affording an at least substantially uniform lighting effect.
Yet another object of the present invention is to provide a vehicle licence plate which affords a high level of retroreflectivity in conjunction with mechanical strength and stability.
In accordance with the principles of the present invention, the foregoing and other objects are attained by a plate such as a motor vehicle registration licence plate, comprising a casing which includes an at least substantially flat rear wall and an edge portion which extends around said rear wall and which projects forwardly therefrom. The edge portion defines a light transmission opening, through which it is possible to see a flat symbol carrier extending parallel to the casing rear wall. The symbol carrier has regions of varying transparency to represent the symbols thereon. To provide illumination between the rear wall and the symbol carrier, the plate has an electrically activatable light foil or film which shines light at least through the regions of high transparency of the symbol carrier, from the rear thereof. Arranged in front of the light film and in parallel relationship therewith is a further film which extends at least over a considerable part of the area of the light transmission opening and which is at least partially transparent for the light coming from the rear from the light film and which is retroreflective for light passing from the exterior through the light transmission opening and incident thereon. The plate further includes a light cover which closes the light transmission opening and which is transparent at least in a region-wise manner. A rough layer is provided between the inside of the light cover and the surface that is theretowards of the retroreflective film.
It will be noted in relation to this structure that plastic materials are available on the market, for example the film offered by 3M under the trade mark 3M SCOTCHLITE, which are substantially transparent in relation to light which impinges on the rear side thereof while they provide for retroreflection of light incident on their front side. As however those films do not enjoy any inherent stiffness they cannot serve as a closure panel to close the light transmission opening of the housing, as is the case with the symbol carrier of German utility model No 297 12 954.6 as discussed hereinbefore.
Therefore, in order to stabilise the electrically activatable light film and the retroreflective film arranged in front thereof, in terms of the flat contact thereof against the inside surface of the rear wall of the casing, it is necessary for the light transmission opening to be closed by a light cover which is transparent in relation to light in both directions and which thus primarily performs a mechanical holding function.
Without further measures being entailed in this respect however the problem which then arises is that the retroreflective film, in regard to its flat contact, bears against the inside of the light cover in an unevenly close relationship therewith over the entire surface area and is in part even sucked thereagainst so that, even when the system enjoys uniform illumination from the inside or the outside, it nonetheless still involves zones of varying lightness and darkness, or the formation of Newton""s rings. That is not only unattractive but it is also inadmissible in particular in the case of motor vehicle licence plates. In accordance with the invention, to overcome those difficulties, a rough layer is provided between the inside of the light cover and the surface that is theretowards of the retroreflective film.
The rough layer can be implemented in different ways. Thus, in accordance with a preferred feature of the invention, arranged between the retroreflective film and the light cover is a further film which is rough on both sides and which enjoys very high transparency in both directions therethrough and which prevents the retroreflective film from being able to bear against the inside surface of the light cover so snugly that the above-mentioned optical interference effects occur. The retroreflective film can then have a completely smooth surface and the inside surface of the light cover can also be smooth.
As an alternative to that additional rough film it is also possible for the rough layer in accordance with the invention to be afforded by a rough coating either on the outwardly facing surface of the retroreflective film and/or on the inwardly facing surface of the light cover.
A particular advantage of the plate according to the invention is that a separate symbol carrier is now no longer absolutely necessary. On the contrary, the symbols to be represented on the plate can be provided on any of the surfaces which, as viewed by a person viewing the plate, are in front of the rear surface of the retroreflective film, that is to say on the front side of the retroreflective film, the rear side or the front side of the film which is rough on both sides, or the front side or the rear side of the light cover. In that respect there is no need for all symbols which are to be represented to be disposed on the same surface.
In another preferred feature of the invention the symbols of the symbol carrier can be provided on one or more of the film surfaces as it is very easy to print the symbols thereon.
In accordance with yet another preferred feature of the invention the films can bear loosely against each other between the rear wall of the casing and the light cover, and can be held by the rear wall and the light cover, thereby making it unnecessary to provide any additional fixing means.
Besides the above-mentioned films, it is also possible to use any other films which have the required optical properties, that is to say which enjoy a level of transparency which is as high as possible. The significant consideration in regard to the retroreflective surface of the film in question is that, independently of the angle of incidence, it reflects light coming from the exterior back in the respective direction of incidence to a very high degree.
The electrically activatable light film may involve an electroluminescence film which is operated in the usual manner by an ac voltage which, in the case of a motor vehicle licence plate, can be produced from the onboard dc voltage system by means of a suitable power supply device.
In a particularly preferred configuration however the electrically activatable light film is in the form of a film with organic light emitting diodes, referred to as an OLED film, which affords the advantage that it can be operated with a dc voltage which can be derived from an onboard dc voltage by an extremely simple ancillary circuit. The operating electronics required for an electroluminescence film can be omitted, as well as particular insulating measures. A further advantage of OLED films is the substantially higher level of luminous density which can be achieved in that way.
Further objects, features and advantages of the invention will be apparent from the description hereinafter of a preferred embodiment.